The present invention relates to the laser processing or ablation of materials, and is suitable, for example, for surgical and medical applications, including operations for correcting refractive errors of the eye, such as photorefractive keratectomy (PRK) and laser in-situ keratomileusis (LASIK). Other examples include medical processes on a wide variety of biological tissue such as retinal tissue, bone or teeth.
Excimer gas lasers have an operating wavelength of 193 nm in the ultraviolet (UV) region of the electromagnetic spectrum. These lasers process material through photo-ablation, vaporising the material while causing little thermal damage to adjacent areas. This property and the availability of these lasers has led to their widespread use in the medical field. However, an all solid state UV laser has been sought as an alternative, owing to a number of inherent disadvantages associated with the excimer laser. These disadvantages include large size and high operating and maintenance costs. Excimer lasers also require the use of an extremely toxic gas.
Solid state lasers offer a smaller, more efficient, less dangerous alternative to excimer gas lasers. These lasers utilize rare-earth elements contained in glass or crystal matrices such as yttrium aluminum garnet (YAG), or yttrium lithium fluoride (YLF). Excitation of the laser medium results in stimulated atoms of elements such as neodymium, erbium and holmium producing high energy laser emissions. A variety of wavelengths may be produced depending on the rare earth element that the laser contains. Some of the more common solid state lasers are Nd:YLF at 1.053 microns, Ho:YAG at 2.1 microns and Er:YAG at 2.94 microns. A Neodymium:YAG laser produces a wavelength of 1064 nm (1.06 microns), which is in the infra-red portion of the electromagnetic spectrum.
Solid state lasers produce beams of longer wavelengths than the excimer laser and have been successfully applied to different medical and industrial processes. However, the longer infra-red wavelengths may also produce undesirable effects when applied to certain materials, such as corneal tissue. As such, a demand exists for a solid state laser source that emits a wavelength in the ultraviolet region.
With the development of new non-linear optical (NLO) crystals, an all solid state UV laser source has been realized. The use of non-linear optical crystals for frequency conversion of high intensity laser emissions is well known to those with an understanding of the art (see, for example U.S. Pat. No. 5,144,630). When an infra-red laser beam is directed through a NLO crystal, its wavelength can be altered. This property allows conversion of an infra-red laser, such as the Nd:YAG at 1064 nm, to a shorter wavelength of 532 nm, a process known as harmonic generation (see, for example, U.S. Pat. No. 5,592,325 and U.S. Pat. No. 4,346,314). Generation of the fourth and fifth harmonic wavelengths of a Nd:YAG laser, at 266 nm and 213 nm respectively, extends the sphere of the solid state laser, making it suitable for a wider range of applications.
Prior art techniques for harmonic generation have often involved the use of non-linear optical crystals of the borate family. Crystals such as beta barium borate (xcex2-BaB2O4 or BBO), lithium borate (LBO), MBeBO3F2 and CsB3O5 have been used previously as frequency conversion compounds (Mori et al 1995 xe2x80x9cNew nonlinear optical crystal: Cesium Lithium Borate. Applied Physics Letters 67(13):1818-1820). Other popular NLO crystals for harmonic generation include Potassium Titanyl Phosphate, (KTP or KTiOPO4) (see, for example, U.S. Pat. No. 5,144,630 and U.S. Pat. No. 5,592,325). However, these crystals exhibit poor energy conversions for fourth and fifth harmonic generation.
More recently with the invention of the NLO crystal, caesium lithium borate (CsLiB6O10 or CLBO), improved performance has been observed in generating the fourth and fifth harmonics of the Nd:YAG laser (Yap et al. 1996 xe2x80x9cHigh-power fourth- and fifth-harmonic generation of a Nd:YAG laser by means of a CsLiB6010.xe2x80x9d Optics Letters 21(17): 1348-1350). Lago et al, (1988, xe2x80x9dCoherent 70.9-nm radiation generated in Neon by frequency tripling the fifth harmonic of an Nd:YAG laser. Optics Letters 13(3): 221-223) were able to generate 20 mJ in a 5 ns pulse at the fifth harmonic, using three BBO crystals for fifth harmonic generation of a Nd:YAG laser at 213 nm. This corresponds to an overall conversion efficiency of 2.4% in terms of input energy at 1064 nm. In comparison, Yap et al, as reported in the aforementioned paper, were able to achieve an overall conversion efficiency of 10.4% using CLBO crystals.
The advantages of using the CLBO crystal over BBO crystals can also be seen by comparison of the non-linear properties of the crystals. When generating harmonic wavelengths in the UV spectrum, CLBO, despite having a smaller non-linear coefficient, has a larger angular bandwidth, spectral bandwidth and temperature acceptance. Also, unlike BBO, CLBO does not suffer from any problems with absorption and/or photorefraction. These features make the crystal useful for medical applications, as it makes the alignment of the laser beam less critical and more stable. In addition, the walkoff angle for CLBO is up to three times smaller than for BBO. CLBO therefore offers an attractive advance over the prior art for fourth and fifth harmonic generation of a reliable solid state laser.
The practical difficulty, however, is to achieve a consistent and reliable laser energy output in the course of a surgical procedure, and from procedure to procedure: CLBO crystals are not inherently robust and stable. It is an object of the present invention to at least in part overcome this difficulty.
The conventional view in utilizing non-linear optical materials for frequency conversion is that their relatively low conversion efficiencies and limited damage thresholds were best countered by pulsing the input laser energy at relatively high frequencies, eg. in the order of the kHz. Furthermore, to guard against overheating of the crystals, resulting in crystal damage, pulse energies were customarily kept low.
The present inventors have appreciated, in one or more embodiment of the invention, that an enhanced outcome can be achieved by maintaining the non-linear optical material at an elevated temperature to stabilise the material, and by addressing the conversion efficiency issue by instead pulsing the source laser beam at a higher pulse energy but lower frequency, selecting the frequency so that the acceptance angle of the non-linear crystals remains, or returns to be, substantially within predetermined limits for successive pulses of the source beam. The laser beam diameter can be expanded or not reduced so much so that the energy density of all laser beams are below the damage thresholds of the crystals.
It is thought that, at the lowered frequency, there is a greater interval between pulses sufficient to allow thermal relaxation for the acceptance angle, even if deviated outside the aforesaid limits, to return to within an acceptable range. By this is meant that the axis or orientation of the acceptance angle returns to within the acceptable range. A satisfactory frequency range is below 100 Hz, preferably between 5 and 50 Hz, more preferably between 5 and 30 Hz, most preferably between 10 and 30 Hz.
In one aspect, the invention provides an apparatus for generating a laser beam of wavelength suitable for ablating material, including:
a source of an initial pulsed laser beam of wavelength unsuitable for said ablation; and
frequency conversion means to derive from the initial laser beam by harmonic generation a laser beam of a wavelength suitable for ablating material;
wherein said initial laser beam is pulsed at a pulse rate between 5 and 30 Hz.
The invention further provides, in another aspect, an apparatus for generating a laser beam of wavelength suitable for ablating material, including:
a source of an initial pulsed laser beam of wavelength unsuitable for said ablation; and
frequency conversion means to derive from the initial laser beam by harmonic generation a laser beam of a wavelength suitable for ablating material;
wherein said frequency conversion means includes a non-linear optical material and means is provided to maintain said material at a temperature of at least 40xc2x0 C.
Corresponding methods are also provided.
The invention further provides, in a further aspect, apparatus for generating a laser beam of wavelength suitable for ablating material, including:
a source of an initial laser beam pulsed at a predetermined pulse rate and of wavelength unsuitable for said ablation; and
frequency conversion means to derive from the initial laser beam a harmonic component of said wavelength suitable for ablating material;
wherein said frequency conversion means includes a non-linear optical material and means is provided to maintain said non-linear material at a predetermined elevated temperature;
and wherein said pulse rate is predetermined whereby the acceptance angle of the harmonic component remains substantially within predetermined limits for successive pulses of the initial laser beam at said elevated temperature.
In a still further aspect, the invention provides a method of generating a laser beam of wavelength suitable for ablating material, including:
providing an initial laser beam pulsed at a predetermined pulse rate and of wavelength unsuitable for said ablation;
directing the initial laser beam through frequency conversion means including a non-linear optical material that derives from the initial laser beam a harmonic component of said wavelength suitable for ablating material;
maintaining said non-linear optical material at a predetermined elevated temperature; and
setting said pulse rate whereby the acceptance angle of the harmonic component remains substantially within predetermined limits for successive pulses of the initial laser beam at said elevated temperature.
In a preferred application, the method includes utilising the laser beam of suitable wavelength to ablate material. The ablated material may be human or animal tissue, including corneal tissue. When the material being ablated is corneal, the UV energy deposited on the material is preferably between 3 and 50 mJ per pulse. A particularly advantageous application is for refractive surgery of the cornea, eg. by photorefractive keratectomy (PRK) or laser in-situ keratomileusis (LASIK).
The aforementioned predetermined limits for the acceptance angle arise from a practical requirement, with applications such as medicine, for the laser to reach stable, desirable energy levels, every time the laser is switched on, without the need for re-alignment of the crystals. Normally, temperature and temperature gradient changes induced in NLO crystals by laser beams cause the acceptance angle of the crystals to shift out of alignment with the beams, usually forcing re-alignment of the crystal orientation. Re-alignment is then needed again once the laser beams have been turned off, before the harmonic components can be restarted.
A preferred angular range defining said predetermined limits is an angle equal to the acceptance angle itself.
The elevated temperature is at least 40xc2x0 C. and preferably greater than 60xc2x0 C. It is thought that there is no upper limit to the temperature, other than in the practical respect that above about 100xc2x0 C,, little additional benefit is achieved in return for the additional heat energy input. The non-linear optical material is preferably held in a heat conductive holder to which heat is applied for maintaining the material at said predetermined elevated temperature. The non-linear optical material is preferably a crystal, and advantageously a pair of juxtaposed crystals for effecting successive frequency conversion operations as the laser beam traverses the crystals in turn. The pair of crystals are advantageously retained together in contact in said holder, means being included to bias the crystals against each other, for minimising energy losses.
The non-linear optical material advantageously includes at least one caesium lithium borate (CsLiB6O10 or CLBO) crystals.
For CLBO crystal, an optimum elevated temperature range is between 60xc2x0 and 200xc2x0 C., most preferably around 80xc2x0 C.
Preferably said method includes directing said beam or a portion of said beam to a laser delivery system and then onto an area of said material to be ablated by means of said laser delivery system. The laser delivery system may include a beam delivery system, a scanning system and/or a fibre optic delivery system. Thus, the laser delivery system includes any system suitable for delivering a laser beam to a desired location.
Preferably the non-linear optical material is in a sealed dry, inert atmosphere.
Preferably said laser beam has a fundamental wavelength of between 0.5 and 2.5 micron, and more preferably approximately 1 micron.
Preferably, the source of the initial laser beam is a solid state laser source, eg. a Nd3+ doped laser medium such as a Nd:YAG, Nd:YLF, Nd:glass or Nd:YVO4 laser source.
Preferably the apparatus includes a beam separating system for separating said laser beam of suitable wavelength from other harmonics generated by the frequency conversion means.